Interpretive Summary: Electrical characteristics, known as dielectric properties, of grain and oilseeds, are closely related to the amount of water that they contain. Consequently, instruments can be designed to sense these dielectric properties and be calibrated to read moisture content. Such instruments, called moisture meters, are widely used in the grain and seed trade for rapid determination of moisture content. Moisture content is especially important in the sale and storage of peanuts so that spoilage and development of toxins can be avoided. If kernel moisture contents of peanut lots exceed 10.5 percent, sale is denied in the United States. Microwave dielectric properties of peanuts are being studied for use in development of microwave moisture meters for use in the peanut industry. In particular, the microwave measurement technique provides a method for reliably measuring the moisture content of the peanut kernels from measurements on the unshelled peanut pods. In these studies, moisture content of kernels from such measurements over a range from 6 to 18 percent moisture content were determined with accuracies between one-half and one percent moisture content. Current moisture meters require the shelling of samples to determine moisture content of the kernels. At peanut buying points, samples go through a thorough grading process before the moisture content is determined, and at that time the decision is made as to whether the peanut lot can be sold. Thus, the new microwave moisture technique offers a real advantage in peanut grading operations, because moisture content can be determined at the beginning of the grading process, and if it is too high, the time and labor lost in grading those samples can be avoided. In this new work, dielectric properties of three peanut types, Runner, Virginia, and Valencia, from various growing locations in the United States were measured and moisture calibrations were determined for each type. Then calibrations were developed for three density-independent functions of the dielectric properties and stability and type independence were determined to be satisfactory. A calibration independent of peanut type with accuracies of less than 1 percent moisture content over the moisture range from 6 to 18 percent was developed. Thus, a single calibration can be used for all types of peanuts, which is a real advantage for moisture meter manufacturers and users of such instruments. The new microwave moisture meters offer promise for improving efficiencies of grading and processing as well as preventing losses due to spoilage and will be useful tools for peanut farmers, handlers, and processors, thus providing safe and high quality products for consumers.

Technical Abstract:
A microwave dielectric method was used for nondestructive and instantaneous determination of moisture content in shelled and unshelled peanuts of various types from transmission measurements of their relative complex permittivities in free space at 23 °C between 5 and 15 GHz. Moisture content was estimated, independent of bulk density, with three density-independent calibration functions and compared to standard oven moisture determinations: two of these functions are permittivity-based, and the other is expressed in terms of attenuation and phase shift. The effectiveness and stability of these three functions for individual peanut types and type-independence were evaluated over broad ranges of frequency, moisture content and bulk density. Statistical analysis showed high coefficients of determination in predicting moisture content with individual type calibrations. Statistical analysis also showed high coefficients of determination in predictions with the combined type-independent calibrations. Therefore, by using microwave moisture sensing, calibration equations can be used to accurately predict moisture content in peanuts with insensitivity to type.